1. Field of the Invention
This invention relates to a package structure for a plastic-molded-type semiconductor device suitable for attaining a higher level of device integration.
2. Description of the Prior Art
Combining two or more semiconductor chips in a single packaged unit is very effective in attaining a higher level of device integration. Further, in the case of inferior product yield, a substantial improvement in terms of product yield is achieved by combining good chips as compared to the case where one device having the same area as that of the two or more chips is packaged.
The combining of two semiconductor chips in a single packaged unit can be effected by attaching the chips to either side of a metal plate called a "chip pad". The "chip pad" constitutes a part of a lead frame, the electrodes of the respective chips being connected to the leads through wires, as disclosed in Japanese Patent Unexamined Publication Nos. 62-131555 and 62-8529. Apart from this, there is a method in which lead frames are joined with each other within a single package, as disclosed in Japanese Patent Unexamined Publication No. 62-119952, and a method in which a two-staged lead frame is plastic-molded, as disclosed in Japanese Patent Unexamined Publication No. 63-124450. Further, although it is difficult to regard it as one packaged unit, there is a method in which two packaged units are joined together, substantially constituting a "single" unit, as disclosed in Japanese Patent Unexamined Publication No. 62-16552.
These prior-art techniques, however, have not been put into practical use because of the following problems:
In the structure in which the chips are mounted on either side of a chip pad, the wire bonding of the chip electrodes to the lead frame is first performed on one side, and then, must be done on the other side. In this process, it is very difficult to effect the wire bonding on the second side without damaging the wires used on the first side.
The problem with the method in which lead frames are joined with each other within a single package is that it requires an excessively large receptacle.
The plastic-molding of a two-staged lead frame is very difficult to perform. No technique for this has yet been developed yet utilizing an ordinary transfer mold on a mass-production basis.
It is certainly easy to join two packaged units together into a single unit. This, however, requires a thick package, so the idea is of no great value from the viewpoint of attaining a higher level of device integration.
In the above prior-art techniques, the leads, positioned in the vicinity of the side surfaces of the semiconductor chips, are connected to the electrodes of these chips through wires, so that the positions of the electrodes are restricted to the peripheral sections of the chips. As a result, these prior-art techniques cannot be applied to semiconductor chips which are to be used in encasing a structure in which the leads extend up to the upper surface of the chip (hereinafter referred to as the "LOC (Lead-On-Chip) structure"), as shown in FIG. 17. That is, these techniques cannot be applied in a case where two semiconductor chips whose electrodes are provided are to be encased into a single unit.
Further, when connecting leads, positioned in the vicinity of the side surfaces of the chips to the electrodes through wires, a suitable dimension is needed between the side surfaces of the chips and those of the package, with the result that the encasement of relatively large-sized chips cannot be effected efficiently. In addition, no consideration has been given in these known examples to a reduction in package thickness.